Construct PerfettoTracedProcess before we access the SystemProducer.

This fixes the cast_linux and cast_audio_linux CI builders since on cast
DETACH_FROM_SEQUENCE compiles to a no-op, and thus the PostTask which
accesses the SystemProducer would execute before we had constructed
PerfettoTracedProcess (which posts a task to  set the SystemProducer).

TBR=eseckler@chromium.org

Bug: 1007299
Change-Id: I64da7192406bb067559a26c44a617a772849fe52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1826789Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Commit-Queue: Stephen Nusko <nuskos@chromium.org>
Auto-Submit: Stephen Nusko <nuskos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700492}

services/tracing/public/cpp/perfetto/perfetto_traced_process.cc

@@ -149,11 +149,17 @@ void PerfettoTracedProcess::ResetTaskRunnerForTesting(
   // call to Get()) which would then PostTask which would create races if we
   // reset the task runner right afterwards.
   DETACH_FROM_SEQUENCE(PerfettoTracedProcess::Get()->sequence_checker_);
+  // Call Get() explicitly. This ensures that we constructed the
+  // PerfettoTracedProcess. On some tests (like cast linux) the DETACH macro is
+  // compiled to nothing, which woud cause this PostTask to access a nullptr the
+  // producer requires a PostTask from inside the constructor.
+  PerfettoTracedProcess::Get();
   PerfettoTracedProcess::GetTaskRunner()->GetOrCreateTaskRunner()->PostTask(
       FROM_HERE, base::BindOnce([]() {
-        PerfettoTracedProcess::Get()
-            ->SystemProducerForTesting()
-            ->ResetSequenceForTesting();
+        auto* producer =
+            PerfettoTracedProcess::Get()->SystemProducerForTesting();
+        CHECK(producer);
+        producer->ResetSequenceForTesting();
       }));
 }